Congo's first democratically elected prime minister was abducted and killed in a cold war operation run by British intelligence, according to remarks said to have been made by the woman who was leading the MI6 station in the central African country at the time.

A Labour peer has claimed that Baroness Park of Monmouth admitted to him a few months before she died in March 2010 that she arranged Patrice Lumumba'skilling in 1961 because of fears he would ally the newly democratic country with the Soviet Union.

In a letter to the London Review of Books, Lord Lea said the admission was made while he was having a cup of tea with Daphne Park, who had been consul and first secretary from 1959 to 1961 in Leopoldville, as the capital of Belgian Congo was known before it was later renamed as Kinshasa following independence.

He wrote: "I mentioned the uproar surrounding Lumumba's abduction and murder, and recalled the theory that MI6 might have had something to do with it. 'We did,' she replied, 'I organised it'."

Park, who was known by some as the "Queen of Spies" after four decades as one of Britain's top female intelligence agents, is believed to have been sent by MI6 to the Belgian Congo in 1959 under an official diplomatic guise as the Belgians were on the point of being ousted from the country.

"We went on to discuss her contention that Lumumba would have handed over the whole lot to the Russians: the high-value Katangese uranium deposits as well as the diamonds and other important minerals largely located in the secessionist eastern state of Katanga," added Lea, who wrote his letter in response to a review of a book by Calder Walton about British intelligence activities during the twilight of the British empire.

Doubts about the claim have been raised by historians and former officials, including a former senior British intelligence official who knew Park and told the Times: "It doesn't sound like the sort of remark Daphne Park would make. She was never indiscreet. Also MI6 never had a licence to kill."

Mystery has continued to surround the death of Lumumba, who was shot on 17 January 1961, although Belgian troops were known to have been involved.

Park met Lumumba, the African leader who was to become the short-lived prime minister of an independent Congo. After his successor took power, she was arrested and beaten by his supporters.

She was able to get herself released and sought local UN intervention, securing the release of Britons and other foreigners, for which she was appointed OBE in 1960.